Hello P.,
Currently IHMO there no workarround, this is due to the management of line breaks around the sub-diagram (around '{{' and '}}').
Here is another minimal test, in order to ask the wanted feature of suppress space around sub-diagram:
@startuml
start
:
{{
node node
}}
;
:test4;
@enduml
[Click to see on PlantUML server]
But in my minimal test, if we suppress line breaks, that also OK:
@startuml
start
:{{
node node
}};
:test4;
@enduml
[Click...]
But as soon as there are elements (like string 'loop') between ":" and "{{" that is KO, as:
@startuml
start
:loop{{
node node
}};
:test4;
@enduml
[Click...]
If that can help,
Regards,
Th.